1. Field of the Invention
This invention relates to signal interpolators, and more particularly, to signal interpolators for time base expanding a portion of an input sample signal and deriving interpolated output sample values.
The need for such a signal interpolator arises, for example, in video special effects equipment. The special effects which can be applied to a video signal are well known. Thus, for example, images on a cathode ray tube can be off-set (moved in any direction), scaled (expanded or compressed in size), rolled (rotated in two or three dimensions) and so on. One way of achieving these special effects involves converting an input analog video signal into digitized sample values each having a pixel address, modifying the resulting individual pixel addresses to achieve the required special effect, storing the sample values at the modified pixel addresses in a field memory, and reading from the field memory to derive the sample values for reconversion into the required output analog signal.
In the particular case of scaling, where the image is to be expanded, signal interpolation is required. That is to say, an input sequence (or in two dimensions an input array) of sample values at a given system sample rate needs to be time base expanded. In other words, new sample values need to be calculated and interpolated, so that in effect the sample rate is increased, although the output sample rate is still equal to the system sample rate.
2. Description of the Prior Art
One way of doing this is to convert the digital sample values back to an analog signal and re-sample the analog signal at the new higher sample rate, but this involves substantial hardward, and also difficult filtering problems. Alternatively, if the input sample rate is N and the output sample rate is M, then in theory the required output could be derived by multiplying the sample rate of the input signal by M and then dividing by N. In practice, the product M.times.N is likely to become excessively high, and also there are again difficult filtering problems.
Another alternative is disclosed in Canadian patent specification No. CA-A-1 186 751 and comprises determining the instantaneous time difference between input and output sample times, producing a control signal representative of that difference, and controlling the sample rate translation in dependence on the control signal. This Canadian specification is concerned solely with audio signals, and the method disclosed is not suitable for application to video signals, in particular because of the two dimensional nature of video signals, whereas audio signals are essentially one dimensional. Nor is the disclosure in this Canadian specification appropriate to deal with other problems which arise in connection with video signals; in particular the possibility that the expansion required may approach infinity, and the need to dispense with redundant information.